Nanoparticles are of fundamental scientific and technological importance as basic building blocks in nanotechnology and as components of novel materials. This is due to several unique features: their small size (1–100 nm) means the properties of these materials may be determined by quantum rather than classical physics; the ratio of surface atoms to bulk atoms is large so that surface physics is important in determining the material properties; the surface properties can be modified through self-assembled monolayer coatings; and they form structures in a size range (1–100 nm) that are not readily accessible through other physical or chemical techniques.
A large variety of nanoparticles have been chemically synthesised in the literature. Metallic e.g. Au, Ag, Pd, Pt, Cu, Fe, etc; semiconductor e.g. TiO2, CdS, CdSe, ITO, etc; insulating e.g. SiO2, organic etc; magnetic e.g. Fe2O3, Fe, etc; superconductor etc. Most synthetic procedures are relatively straight forward and many have been developed that yield relatively uniform nanoparticles (±10%) in a range of sizes. The size of the particle is determined by the stoichiometry of the components (the ions and the reducing agent), yielding what are essentially metal (or non-metal, etc depending on the starting materials) cores surrounded by chemical coatings. For example, 15 nm Au nanoparticles (used in the work reported here) can be made by reducing Au3+ ions to metallic Au using sodium citrate as the reducing agent, by the following method: aqueous HAuCl4 solution (1% w/v, 5 ml) is added to 500 ml water, heated to boiling, before adding sodium citrate (1% w/v, 12.5 ml). This is boiled for 15 min then cooled, yielding a wine-red solution of 15 nm gold nanoparticles. The colour of the colloidal solution is due to surface plasmon adsorption (˜520 nm) of the Au nanoparticles. The adsorption frequency varies depending on the size of the nanoparticles and thus can be used to check the average size of the particles. The size of the particles can be varied from 2–60 nm by varying the ratio of HAuCl4 to sodium citrate. Other methods exist for the formation of Au nanoparticles in non-polar solvents, which are required when using water insoluble cross-linking molecules.
Several methods have been reported in the literature for forming bulk nanoparticle materials. Drying down of colloidal gold solution droplets deposited onto substrates have produced ill-defined structures (Schmid, G.; Lehnert, A.; Kreibig, U.; Adamczyk, Z.; Belouschek, P. (1990) Z. Naturforsch., 45b, 989). Electrophoretic deposition has been reported to take advantage of the charge surrounding the nanoparticles in solution but these films are difficult to control, produce cracked films, and require conducting substrates (Giersig, M.; Mulvaney, P. (1993) J. Phys. Chem., 97, 6334). Films have also been produced by cross-linking nanoparticles with linkers, allowing the aggregates to precipitate, then compressing the bulk material into pellets. This method is non-uniform and has limited utility (Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. (1995) Adv. Mater. 7, 795).
Successful conducting gold films made on glass supports by a monolayer by monolayer deposition, have been reported in the literature (Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. (1997) Chem. Mater. 9, 1499). This “layer-by-layer” deposition begins by coating a substrate surface with charged (positive) groups. The substrate is then dipped into a solution of negatively charged Au nanoparticles which adsorb onto the positively charged surface. Due to the negative charge on the nanoparticles only a ˜30% surface coverage is achieved. The dilute nature of the nanoparticle solutions means that this coating step takes 2–24 hours. The sodium citrate providing the negative charge on the nanoparticle is replaced in the next step by a cross-linker molecule which acts as a “glue” between the nanoparticles, e.g. a difunctionalised alkane such as HS—(CH2)2—SH. In this molecule, one sulfur group binds strongly to the gold, displacing the sodium citrate and neutralising the particle, leaving the second sulfur group exposed ready to bind the next layer of Au nanoparticles. This process takes a further 10–100 minutes. In between coatings, the surface must be rigorously washed to prevent precipitation of the nanoparticles. The procedure is repeated for as long as required in order to build up the multilayer structure. Films of 12 or more layers, using a two carbon cross-linker molecule, HS—(CH2)n—OH, have been shown to be conductive. Whilst this method is solution based and so may be automated it suffers from a number of disadvantages; the procedure is slow, can take several days and places a limit on the thickness of the films that can be realistically built, it yields amorphous structures, and uniformity depends on the uniformity of the initial substrate coating.
Previous literature work has reported that the conductivity of bulk nanoparticle materials formed from the above coated nanoparticles depends dramatically on the length or thickness of the cross-linker molecules, and hence the distance between the nanoparticles.
Table 1 gives the approximate values of resistivity for the various cross-linker molecules which have been reported in the literature. Note that these values were measured on nanoparticle materials made using different techniques (e.g. compressed pellets, layer-by-layer assembly, differing Au nanoparticle sizes, different laboratories, etc), hence comparisons should be made cautiously. However, it is apparent that small changes in the interparticle distance (0.4 to 2 nm), as determined by cross-linker length, results in large changes in resistivity (more than 10–12 orders of magnitude).
TABLE 1Resistivity of films fabricated using different length cross-linkingmolecules. Note: caution should be used when comparing this literaturedata as the results were based on bulk materials fabricated using adifferent techniques.Technique usedLinkerLinker lengthResistivityto form bulk materialHS(CH2)16SH~2nm~109Ω cmDrop-cast filmaHS(CH2)6SH~1nm~10Ω cmCompressed pelletbHS(CH2)2OH~0.4nm~10−4Ω cmLayer-by-layercbulk gold—~10−6Ω cmaTerril, R. H. et al (1995) J. Am. Chem. Soc., 117, 12537bBrust, M. et al (1995) Adv. Mater., 7, 795cMusick, D. et al (1997) Chem. Mater., 9, 1499.
Some models are just beginning to appear in the recent literature regarding the nature of the conduction observed in the bulk materials formed using nanoparticles. The size and large surface area of the nanoparticles determine the physics of these particles. When the wavelength of the electrons approaches the same order as the particle size, quantum mechanical rules apply. There are only a few published reports which discuss the behaviour of bulk materials formed from nanoparticles. Quantum confinement has been discussed for metal particles between 1 and 10 nm in size, and for semiconductor nanoclusters of larger sizes due to different conditions in the bulk (Schmid, Baumle, Geerkens, Heim, Osemann, Sawitowski (1999) Chem. Soc. Rev., 28, 179–185). A direct relationship between the gold nanoparticle spacing created by cross-linker molecules and the activation energy needed to start electronic tunneling via the linkers, from one nanoparticle to the next, has been reported (G. Schmid & L. F. Chi, (1998), Adv. Mater., 10, 515). Single-electron tunneling has been observed in individual single gold particles coated with a 3 nm thick self-assembled monolayer when measured using an STM tip (Yau, S.-T., P. Mulvaney, W. Xu, G. M. Spinks (1998) Phys. Rev. B, 57, 124–127). A report describing a “single electron transistor” discusses electron conduction exhibiting a Coulomb staircase occurring through the self-assembled monolayers of gold particle aggregates which were used to bridge a gap between the source and drain structures on a silicon dioxide substrate (T. Sato, H. Ahmed, D. Brown, B. Johnson (1997) J. Appl. Phys. 82, 696–701).